David Marcus, president of eBay’s PayPal payments subsidiary, is leaving to become Facebook’s vice president of messaging products.

At Facebook, Marcus will run Facebook Messenger, a standalone app and a feature within Facebook’s desktop platform. But he won’t oversee WhatsApp, the mobile-messaging service Facebook agreed to buy for $19 billion in February, a Facebook spokeswoman said.

The high-profile hire is the latest example of Facebook’s push into mobile messaging, an increasingly important part of its plan to connect the world through its platform.

Facebook Messenger has a fraction of WhatsApp users, and trails far behind other popular services like Line and WeChat in many countries. Facebook said 12 billion messages are sent through its messenger service every day and around 200 million people use it every month. By contrast, WhatsApp has more than 500 million monthly users.

In April, Facebook began forcing mobile users to download Messenger to send and read messages from other users. Facebook had previously included that feature in its main mobile app.

Marcus and other senior eBay managers spent much of the early part of the year defending PayPal from activist investor Carl Icahn, who said the payments unit should be spun off. Icahn ultimately relented, agreeing to insider status at the company. An eBay spokeswoman said Icahn had no input on Marcus’s departure.

It’s unclear why Facebook is putting so many resources in its own messaging service when it paid such a steep price to acquire another one.

Last week, WhatsApp co-founder Brian Acton called the Facebook Messenger “separate but equal” from WhatsApp, but didn’t explain what he meant.

Facebook founder and chief executive Mark Zuckerberg has said the apps are used differently. Facebook Messenger is used by people to stay in touch with friends and relatives, who may not expect an immediate response. On the contrary, WhatsApp is used for more instant communication, as a replacement for text-messaging conversations.

Marcus said Zuckerberg shared his “compelling vision” about mobile messaging, according to his own Facebook page. That vision “won me over” Marcus said.

The eBay spokeswoman said Marcus’s decision to leave was his own. She said the change is effective at the end of June, after which eBay CEO John Donahoe will lead the division on an interim basis. EBay said the company will look inside and outside the company for a replacement.

In a statement, Donahoe said Marcus helped with “reinvigorating product design and innovation” at PayPal.

Marcus said he felt comfortable leaving PayPal after two years at the helm because the company is in good shape. “PayPal is on track to achieve the greatness it deserves in the years to come,” he said.